Datasheet AD8231 (Analog Devices) - 19

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Data Sheet AD8231 LAYOUT INCORRECT CORRECT +VS +VS
The AD8231 is a high precision device. To ensure optimum performance at the PCB level, care must be taken in the design of the board layout. The AD8231 pinout is arranged in a logical manner to aid in this task.
AD8231 AD8231 REF REF Power Supplies
The AD8231 should be decoupled with a 0.1 μF bypass capacitor between the two supplies. This capacitor should be placed as close
–VS –VS
as possible to Pin 11 and Pin 12, either directly next to the pins or
TRANSFORMER TRANSFORMER
beneath the pins on the backside of the board. The auto-zero
+V +V
architecture of the AD8231 requires a low ac impedance between
S S
the supplies. Long trace lengths to the bypass capacitor increase this impedance, which results in a larger input offset voltage. A stable dc voltage should be used to power the instrumentation
AD8231 AD8231
amplifier. Noise on the supply pins can adversely affect
REF REF
performance.
10MΩ Package Considerations –VS –VS
The AD8231 comes in a 4 mm × 4 mm LFCSP. Beware of
THERMOCOUPLE THERMOCOUPLE
blindly copying the footprint from another 4 mm × 4 mm
+VS +VS
LFCSP part; it cannot have the same thermal pad size and leads.
C C
Refer to the Outline Dimensions section to verify that the PCB symbol has the correct dimensions. Space between
1 R f
the leads and thermal pad should be kept as wide as possible
AD8231 HIGH-PASS = 2πRC AD8231 C C
for the best bias current performance.
REF REF Thermal Pad R
The AD8231 4 mm × 4 mm LFCSP comes with a thermal pad.
–V
3
S –VS
-03 86 This pad is connected internally to −V
CAPACITIVELY COUPLED CAPACITIVELY COUPLED
65 S. The pad can either be 0 left unconnected or connected to the negative supply rail. For Figure 51. Creating an IBIAS Path high vibration applications, a landing is recommended.
INPUT PROTECTION
Because the AD8231 dissipates little power, heat dissipation All terminals of the AD8231 are protected against ESD. In is rarely an issue. If improved heat dissipation is desired (for addition, the input structure allows for dc overload conditions example, when ambient temperatures are near 125°C or when a diode drop above the positive supply and a diode drop below driving heavy loads), connect the thermal pad to the negative the negative supply. Voltages beyond these limits cause the ESD supply rail. For the best heat dissipation performance, the diodes to conduct and current to flow. If overvoltage events are negative supply rail should be a plane in the board. See the anticipated, an external resistor should be used in series with Thermal Resistance section for thermal coefficients with and each of the inputs to limit the current to below 10 mA. Currents without the pad soldered. up to 100 mA can be sustained for a few seconds.
INPUT BIAS CURRENT RETURN PATH
Note that if either input is brought below the negative supply The input bias current of the AD8231 must have a return path to the point where the ESD diode turns on, the AD8231 output to common. When the source, such as a thermocouple, cannot can phase-reverse. provide a return current path, one should be created, as shown in Figure 51. Rev. C | Page 19 of 24 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE MAXIMUM POWER DISSIPATION ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS INSTRUMENTATION AMPLIFIER PERFORMANCE CURVES OPERATIONAL AMPLIFIER PERFORMANCE CURVES PERFORMANCE CURVES VALID FOR BOTH AMPLIFIERS THEORY OF OPERATION AMPLIFIER ARCHITECTURE GAIN SELECTION REFERENCE TERMINAL LAYOUT Power Supplies Package Considerations Thermal Pad INPUT BIAS CURRENT RETURN PATH INPUT PROTECTION RF INTERFERENCE COMMON-MODE INPUT VOLTAGE RANGE REDUCING NOISE APPLICATIONS INFORMATION DIFFERENTIAL OUTPUT MULTIPLEXING USING THE AD8231 WITH BIPOLAR SUPPLIES SALLEN KEY FILTER OUTLINE DIMENSIONS ORDERING GUIDE AUTOMOTIVE PRODUCTS